For the quantitative analysis of small amounts of gaseous substances several methods are used at present, such as spectrophotometry, fluorimetry, gas-chromatography and galvanometry. All of these require relatively expensive equipment. Particularly in cases where an accurate determination of very small amounts of i.e. gaseous pollutants (with hygienic limits set at 1 ppm or lower) is required, the available instruments tend to be both costly and bulky. Due to a growing demand for a better working environment, a need has long been felt for reliable, simple, cheap and in particular, sensitive measuring devices for the quantitative determination of gaseous or gasborne pollutants with concentrations ranging from 10 ppm down to thousandths of ppm.
It is known (i.e. Anal. Chem., 45, 443A (1973)) that a large number of gaseous substances can be accurately determined by means of chemiluminescent reactions. Such reactions generate light, the intensity of which is directly proportional to the concentration of the reacting substances. If during the reaction the luminescent substance is in excess, the light intensity is directly proportional to the concentration of the substance investigated in the sample; thus a simple measurement of light intensity yields an accurate determination of the sample concentration. The radiation can be generated in any of the visible, ultraviolet and infrared spectral regions.
Generally, a luminescent substance can be either a solid, a liquid or a gas. When a gaseous sample reacts with a liquid reagent, the light generation will be proportional to the area of the gas-liquid interface.
In a known device this is achieved by adsorbing the reagent on a gel and passing the gaseous sample over the surface of the gel (Geophys. Res. 65, 3975 (1960)). The disadvantage of this method (as that of other devices in which a gaseous sample is bubbled through a liquid reagent) is the relatively small size of the gas-liquid interface. When the objective is to continuously determine a gaseous pollutant such as ozone, continuous blowing of the sample against the reagent in the gel or bubbling it through the reagent causes the latter to successively change its concentration as a result of evaporation and reaction with the sample, resulting in an unsatisfactory accuracy of the analysis.
If both reagent and sample are gaseous a number of known devices are available where both gases mix and react to yield chemiluminescence. In the British Pat. Nos. 1,341,346 and 1,353,722 two different reaction chambers are described, in which the gases are mixed in front of a photomultiplier. These devices can only be used for purely gaseous components.
In the American Pat. No. 3,998,592 a thermoelectric heat-pump is used for simultaneous heating of the reaction cell and cooling of the photomultiplier. This device is also intended for gaseous components only. Furthermore, the thermoelectric heat-pump only yields a certain preset temperature difference between the hot and the cold side. As a rule, the photomultiplier has to be cooled further--either directly or by extracting more heat from the hot side of the thermoelectric heat-pump. With some bio-or chemiluminescent systems better light yields are obtained at temperatures below ambient temperature; in such cases the above-mentioned device is not applicable.